1. Technical Field
The present invention is directed to a temperature measurement circuit, and, in particular, to a thermocouple measurement circuit employed in a control circuit for an electrosurgical generator.
2. Description of the Related Art
Electrosurgical generators are employed by surgeons in conjunction with an electrosurgical tool to cut, coagulate, desiccate and/or seal patient tissue. High frequency electrical energy, e.g., radio frequency (RF) energy, is produced by the electrosurgical generator and applied to the tissue by the electrosurgical tool. Both monopolar and bipolar configurations are commonly used during electrosurgical procedures.
Electrosurgical generators typically include power supply circuits, front panel interface circuits, and RF output stage circuits. Many electrical designs for electrosurgical generators are known in the field. In certain electrosurgical generator designs, the RF output stage can be adjusted to control the RMS (root mean square) output power. The methods of controlling the RF output stage may include changing the duty cycle, or changing the amplitude of the driving signal to the RF output stage. The method of controlling the RF output stage is described herein as changing an input to the RF output stage.
Electrosurgical techniques have been used to seal or fuse small diameter blood vessels, vascular bundles and tissue. In this application, two layers of tissue are grasped and clamped together while electrosurgical power is applied. By applying a unique combination of pressure, gap distance between opposing seal surfaces and controlling the electrosurgical energy, the two tissue layers are welded or fused together into a single mass with limited demarcation between tissue layers. Tissue fusion is similar to vessel sealing, except that a vessel or duct is not necessarily sealed in this process. For example, tissue fusion may be used instead of staples for surgical anastomosis. Electrosurgical power has a desiccating effect on tissue during tissue fusion or vessel sealing.
One of the issues associated with electrosurgical sealing or fusion of tissue is undesirable collateral damage to tissue due to the various thermal effects associated with electrosurgically energizing tissue. The tissue at the operative site is heated by electrosurgical current typically applied by the electrosurgical instrument. Healthy tissue adjacent to the operative site may become thermally damaged if too much heat is allowed to build up at the operative site or adjacent the sealing surfaces. For example, during sealing, the heat may conduct or spread to the adjacent tissue and cause a significant region of tissue necrosis. This is known as thermal spread. Thermal spread becomes important when electrosurgical instruments are used in close proximity to delicate anatomical structures. Therefore, an electrosurgical generator that reduced the possibility of thermal spread would offer a better opportunity for a successful surgical outcome.
Another issue associated with electrosurgical tissue sealing or tissue fusion is the buildup of eschar on the surgical instrument. Eschar is a deposit which is created from tissue that is charred by heat. Surgical tools often lose effectiveness when coated with eschar.
Conventional electrosurgical systems have employed temperature sensors in the surgical tool to monitor conditions at the operative site and/or the temperature of the tissue being manipulated. An exemplary temperature sensor used in such systems is a thermocouple due to its small size and low cost. However, thermocouples alone are not accurate and required compensation circuitry to achieve a desired level of accuracy.
Therefore, it would be desirable to have a temperature measurement circuit for an electrosurgical system for accurately determining a temperature of an operative site and/or tissue of a patient. Furthermore, it would be desirable to have a temperature measurement circuit for controlling an electrosurgical generator for producing a clinically effective output and, in addition, reducing the amount of heat and thermal spread at the operative site.